Echo cancellation in two-wire transmission path repeaters

ABSTRACT

A two-wire transmission path repeater includes, between two hybrid circuits coupled to the path, an echo canceller and an amplifier for each direction of transmission, each echo canceller including a transversal filter and a subtracting circuit. The power ratio between signals before and after the subtracting circuit of each echo canceller is used to distinguish between simplex and duplex signals. If the power ratio exceeds a given value, which is dependent upon the input signal power, for only one transmission direction then a simplex signal in the opposite direction is determined, the gain of the amplifier for the one transmission direction is reduced, and adaptation of the transversal filter coefficients for the opposite direction is prevented. If the power ratio for both directions falls below the given value then a duplex signal is determined, the gain of both amplifiers is increased and adaptation of the coefficients of both transfer filters is continued for a short period, then the gain of both amplifiers is reduced and adaptation prevented, thereby facilitating stable operation of the repeater.

This invention relates to echo cancellation, and is particularlyconcerned with the operation of echo cancellers in repeaters in 2-wirebidirectional transmission paths.

It is well known to use echo cancellation techniques to reduce echoestypically arising from hybrid junctions between 2-wire and 4-wiretransmission paths. For example, Kelly, Jr. et al. U.S. Pat. No.3,500,000 entitled "Self-Adaptive Echo Canceler" illustrates echocancellers, each comprising a transversal filter and a subtractingcircuit, provided at opposite ends of a 4-wire transmission pathadjacent to respective hybrid junctions to 2-wire transmission paths.

In order to provide for amplification and equalization of signals, suchas voice frequency signals, transmitted in both directions via a 2-wiretransmission path, it is known to provide a repeater includingequalizing amplifiers for signals in the two transmission directions. Asthe amplifiers operate as 4-wire devices, i.e. separately for the twotransmission directions, such a repeater also includes hybrid circuitsfor converting between the 2-wire and 4-wire paths, and echo cancellersfor cancelling the local echoes produced by these hybrid circuits. Thussuch a repeater has the general configuration of a 4-wire transmissionnetwork as illustrated in the above-mentioned patent, except that thelong-distance, and consequently attenuating, 4-wire path of such networkis replaced by local connections via amplifiers providing gain.

The presence of amplifiers providing gain in the signal paths betweentwo echo cancellers in such a repeater gives rise to stability problemsin certain operating conditions in the presence of simplex (one-wayonly) and full duplex (simultaneous both-way) signals. In particular,such problems arise from a divergence of the coefficients of thetransversal filters, away from their optimum values, under the influenceof residual echo signal components.

An object of this invention, therefore, is to provide improved repeaterapparatus and improved methods of operation of such apparatus.

According to one aspect this invention provides apparatus comprising:hybrid circuit means for coupling a two-wire transmission path tofour-wire transmit and receive paths; echo cancelling means in thefour-wire transmit path for cancelling echoes on the transmit path ofsignals on the receive path; amplifying means for amplifying signals onthe transmit path; and control means responsive to the power ratiobetween signals on the transmit path before and after the echocancelling means for controlling the gain of the amplifying means.

According to another aspect of this invention there is provided arepeater for a 2-wire bidirectional transmission line, the repeatercomprising: two unidirectional transmission paths, one for eachtransmission direction, each comprising echo cancelling means andamplifying means; two hybrid circuits for coupling the unidirectionaltransmission paths in the 2-wire transmission line; and control meansresponsive to the power ratio between signals on each unidirectionaltransmission path before and after the echo cancelling means forcontrolling the gain of each amplifying means.

In an embodiment of the invention the control means is responsive to theratio of the power of signals on either one of the unidirectionaltransmission paths before the echo cancelling means to the power ofsignals on the same unidirectional transmission path after the echocancelling means exceeding a given value to reduce the gain of theamplifying means in that unidirectional transmission path and, whereeach echo cancelling means comprises a transversal filter havingself-adaptive filter coefficients, to prevent adaptation of the filtercoefficients of the transversal filter of the echo cancelling means ofthe other unidirectional transmission path.

Preferably the control means is also responsive to the ratio of thepower of signals on each unidirectional transmission path before theecho cancelling means to the power of signals on the same unidirectionaltransmission path after the echo cancelling means both falling below agiven value to increase the gain of both amplifying means for apredetermined period, and for allowing adaptation of the filtercoefficients of both transversal filters during the predetermined periodduring which the gain of both amplifying means is increased, and forpreventing adaptation of the filters coefficients of both transversalfilters and decreasing the gain of both amplifying as the end of thepredetermined period.

The invention will be further understood from the following descriptionwith reference to the acompanying drawings, in which:

FIG. 1 schematically illustrates in block diagrammatic form parts of arepeater which operates in accordance with this invention; and

FIGS. 2 and 3 are state diagrams illustrating operating states of therepeater of FIG. 1.

Referring to FIG. 1, parts of a 2-wire repeater for voice frequencysignals are shown coupled between 2-wire bidirectional transmissionlines 10 and 12. For convenience of explanation herein, signalstransmitted from the line 10 to the line 12, i.e. from left to right asillustrated in FIG. 1, are referred to as signals of a left channel, andsignals transmitted in the opposite direction from right to left asillustrated are referred to as signals of a right channel.

For the left channel, signals from the line 10 are supplied via a hybridcircuit 14 and a coder 16, and via a summer 18 of a left echo cancellerwhich also comprises a left transversal filter 20, to an equalizingamplifier 22 which has a gain Gl (at a given frequency of for example 1kHz). Equalized and amplified left channel signals from the output ofthe amplifier 22 are supplied via a decoder 24 and a hybrid circuit 26to the line 12. The coder 16 and decoder 24, commonly referred totogether as a codec, serve in known manner to convert the left channelsignals between a digital form for handling within the repeater and ananalog form for transmission on the lines 10 and 12.

Conversely, signals for the right channel are supplied from the line 12via the hybrid circuit 26, a coder 28, a summer 30 which forms with aright transversal filter 32 a right echo canceller, an equalizingamplifier 34 having a gain Gr, a decoder 36, and the hybrid circuit 14to the line 10.

The left echo canceller serves to cancel echoes of the right channelsignals which pass to the left channel via the hybrid circuit 14. Tothis end, the right channel signal is filtered by the left transversalfilter 20 the output of which is supplied to a subtracting (-) input ofthe summer 18, coefficients of the left transversal filter 20self-adapting in known manner in dependence upon the left channel signalat the output of the summer 18, supplied to the filter 20 via a line 38.Similarly, echoes of the left channel signal are cancelled by the rightecho canceller, adaptation of the coefficients of the right transversalfilter 32 being dependent upon the right channel signal at the output ofthe summer 30 and supplied to the filter 32 via a line 40.

In addition to the known arrangement of components as described above,the repeater includes a control circuit 42 and detectors 44, 46, 48, and50. The detectors 44 to 50 serve to monitor the signal power ofrespectively the left channel before and after the summer 18 and theright channel before and after the summer 30, these signal powers beingreferred to as Tl, Tl', Tr, and Tr' respectively. The control circuit 42is responsive to the outputs of the detectors 44 to 50 in the mannerdescribed in detail below, and serves to control the gains of theamplifiers 22 and 34 and to inhibit or allow adaptation of thecoefficients in the transversal filters 20 and 32. Inhibition of theadaptation of the coefficients corresponds to a freezing or holding ofthe coefficients at their current values, and can be considered to beequivalent to opening the line 38 or 40 to remove the control signal foradaptation in the respective transversal filter.

The above description with respect to FIG. 1 relates to a repeater inaccordance with the invention in the form of discrete components, thisbeing preferred for a clear explanation of the invention. In a preferredimplementation of this embodiment of the invention, however, all of thecomponents shown within a broken line enclosure 52 in FIG. 1 areconstituted by a single mask programmed digital signal processingintegrated circuit device (dsp i.c.), for example the Texas Instrumentsdsp i.c. known by the designation TMS320.

In a known form of repeater, not including the elements 42 to 50 asillustrated in FIG. 1, a problem arises in the case of full duplexsignals, for example data signals, which are simultaneously transmittedin the two directions of transmission. In known repeaters a full duplexsituation is detected using a double-talk detector, and the adaptationof the transversal filter coefficients is prevented while thedouble-talk condition is present. A double-talk decision isconventionally reached when for example the signal power Tl at theoutput of the coder 16 is greater than the signal power Rr at the inputof the decoder 36 minus a predetermined level Z; this fails to produce adouble-talk decision in the event that Tl<Rr-Z (where the values areexpressed in decibels), which is a condition which is quite likely tooccur in repeaters where gain is provided by the amplifiers 22 and 34.In addition, it has been found that in practice merely preventingadaptation of the filter coefficients in response to a double-talkcondition is not sufficient to ensure reliable data transmission in thepresence of gain provided by the amplifiers 22 and 34.

The known form of repeater also exhibits a disadvantage in the case ofsimplex, or one-way, transmission. For example if a signal is present onthe right channel but not on the left channel, then an echo of the rightchannel signal is present on the left channel and is largely removed bythe left echo canceller. However, a residual echo remains at the outputof the summer 18. In the prior art this residual echo has been removedby a center clipper (not shown), but this has the disadvantage ofintroducing a switching effect and removing the desirable subjectivefeeling of presence over the left channel, this being particularlynoticeable due to the gain provided by the amplifier 22. If no centerclipping is used, the residual echo is amplified by the amplifier 22 andhas an adverse effect on the coefficients in the right transversalfilter 32, consequently adversely affecting the transmission path of theright channel signal.

These disadvantages are substantially eliminated by the operation of therepeater illustrated in FIG. 1 with the elements 42 to 50 in the mannerdescribed below with reference to the state diagrams in FIGS. 2 and 3,which relate to full duplex and simplex situations respectively.

In each of these situations, for each channel, a measure of theenhancement of echo return loss due to the respective echo canceller isused as a factor in determining the operating state of the repeater.This echo return loss enhancement, or ERLE, for each channel is theratio of the signal power Tl or Tr, detected by the detector 44 or 48respectively for the channel before the respective summer 18 or 30, tothe signal power Tl' or Tr' detected by the detector 46 or 50respectively for the channel at the output of the respective summer 18or 30. In other words, the ERLE is the ratio Tl/Tl' for the left channeland Tr/Tr' for the right channel. If these signal powers are expressedconventionally in decibels as power ratios, then the ERLE is thedifference in decibels between the respective power ratios.

The advantage of using the ERLE as a factor in determining the operatingstate of the repeater is that it facilitates providing a cleardistinction of full duplex and simplex situations, even in the presenceof gain provided by the amplifiers 22 and 34. For example, in thesimplex situation with a right channel relative signal level of 0 dB atthe output of the amplifier 34 and no left channel signal, the echo ofthe right channel signal appearing on the left channel at the output ofthe coder 16 may be at -3 dB and the left echo canceller may reduce thisto -30 dB at the output of the summer 18. This gives an ERLE of(-3--30=) 27 dB, this value being typical of a simplex situation. If afull duplex situation exists, with the same right channel signal leveland a left channel signal at a relative level of 0 dB, then thedetectors 44 and 46 will both detect power levels of about 0 dB givingan ERLE of 0 dB, this low value being typical of a full duplexsituation. If the left channel signal is at a much lower relative powerlevel, for example -12 dB, then the detector 44 detects a level of about-3 dB due to the echo and the detector 46 detects a level of about -12dB due to the left channel signal, giving an ERLE of (-3--12=) 9 dB,which is still sufficiently small (and clearly distinguishable from the27 dB figure for the simplex case) to indicate a full duplex situation,even though the left channel signal level is much less than the level ofthe echo from the right channel.

Referring now to FIG. 2, three states of the control circuit 42, andhence of the repeater as a whole, are illustrated and are referenced 60,62, and 64. The state 60 is a normal operating state which prevails forexample in the absence of signals in both directions of transmission, inwhich the tranversal filter coefficients adapt themselves inconventional or normal manner and in which the amplifiers 22 and 34 havepredetermined normal gains Gl and Gr respectively. The states 62 and 64relate to full duplex situations as discussed below.

Between the individual states 60, 62, and 64 FIG. 2 illustratesconditions which, if and when satisfied, cause transitions between thestates as shown by arrows in FIG. 2. Thus a transition from the state 60to the state 62 occurs if, for a predetermined period of one secondprovided to distinguish a full duplex situation (typically datatransmission lasting for an extended period of time) from a double-talksituation (typically lasting for a very brief period), the signal powerlevels Tl and Tr detected by the detectors 44 and 48 are both greaterthan a threshold level T and the ERLE for the left and right channelsare both less than a given ratio X.

The threshold level T is selected to be a low level suitable fordistinguishing between signals and noise, and is for example about -40dBm. The ratio X is selected for convenience for distinguishing betweensimplex and duplex situations, and may vary in a predetermined mannerfor each channel in dependence upon the signal power level Tl or Trwhich is present on the respective channel. In an embodiment of theinvention, for each channel the ratio X is selected to be 12 dB for aninput signal power level on the channel of less than -20 dBm and isselected to be 17 dB for an input signal power level on the channel ofgreater than -20 dBm.

From the above description it should be appreciated that the controlcircuit 42 responds to the signal power levels detected by the detectors44 to 50 to reach the state 62 in a full duplex situation. In order toprovide for long term stability of operation of the repeater in thissituation, this state 62 is adapted for only a short predeterminedperiod of, for example, 200 ms, whereupon the control circuit 42 adoptsthe state 64 for the remainder of the duration of the full duplexsituation. As illustrated in FIG. 2, for the 200 ms period that thecontrol circuit 42 has the state 62, it controls each of the amplifiers22 and 34 to have a gain which is 4 dB above the normal gain of therespective amplifier, and allows continued adaptation of thecoefficients of both of the transversal filters 20 and 32. On enteringthe state 64, the control circuit 42 controls the amplifiers 22 and 34to resume their normal gains, (i.e. it decreases the gains of theamplifiers by the same amount of 4 dB) and freezes the coefficients ofboth transversal filters by preventing continued adaptation.

The result of the above control sequence is that, during the 200 msperiod that the state 62 is occupied, the coefficients of thetransversal filters are adapted to stablize the repeater circuit (inparticular the closed loop via the amplifier 22, filter 32, summer 30,amplifier 34, filter 20, and summer 18) at the increased gain of theamplifiers. On entering the state 64, the coefficients are frozen andthe gain of each amplifier is reduced to provide a singing (oroscillation) margin of 4 dB. This margin is sufficient to ensure longterm stability of the repeater.

As also illustrated in FIG. 2, the state 64 is maintained until both ofthe signal power levels Tl and Tr fall below the threshold level T,indicating that the full duplex situation has ended. In this case thenormal state 60 is resumed.

Referring now to FIG. 3, the normal state 60 of the control circuit 42is shown with two states 66 and 68 which correspond to simplextransmission states for signals on respectively the right and leftchannels only. As the state 68, and the transitions between this stateand the normal state 60, are the converse to those for the state 66,merely relating to transmission in the opposite direction, a detaileddescription of these is not necessary.

As illustrated in FIG. 3, a simplex state of a signal on only the rightchannel causes a transition from the normal state 60 to the state 66 bydetection of signal powers Tl (this being an echo) and Tr by thedetectors 44 and 48 which are greater than the threshold level T, by thedetection of an ERLE for the left channel which is greater than theratio X, and by the detection of an ERLE for the right channel which isless than or equal to the ratio X, the threshold level T and the ratio Xhaving the same values as discussed above. In the state 66, the controlcircuit 42 prevents continued adaptation of the transversal filter 32,thereby freezing its coefficients so that they do not diverge from theircurrent values, and reduces the gain of the amplifier 22 by 3 dB inorder to reduce the level of the residual echo signal on the leftchannel. This gain reduction and the freezing of the coefficientsobviate the need for center clipping as in the prior art.

As also illustrated in FIG. 3, the normal state 60 is resumed from thestate 66, the gain of the amplifier 22 being returned to normal andadaptation of the filter 32 being resumed, in response to either theERLE of the left channel falling below the ratio X or both of the signalpower levels Tl and Tr falling below the threshold level T.

Whilst a particular embodiment of the invention has been described indetail, it should be appreciated that numerous modifications,variations, and adaptations may be made thereto without departing fromthe scope of the invention as defined in the claims. In particular, itis observed that the invention is not limited to the particular signalpower levels, gain changes, and time periods discussed above.

What is claimed is:
 1. Apparatus comprising:a transmit path and areceive path; echo cancelling means for cancelling echoes on thetransmit path of signals on the receive path, the echo cancelling meanscomprising a transversal filter having adaptive filter coefficients, theecho cancelling means further having a first input coupled to thetransmit path, a second input coupled to the receive path, and anoutput; substantially linear amplifying means having a controllable gainand having an input coupled to the output of the echo cancelling means;and control means responsive to the power ratio between signals at thefirst input and the output of the echo cancelling means for controllingthe gain of the amplifying means and for controlling adaptation of thefilter coefficients; wherein the control means is responsive to theratio of the power of signals at the first input of the echo cancellingmeans to the power of signals at the output of the echo cancelling meansfalling below a given value to: increase the gain of the amplifyingmeans for a predetermined period; allow adaptation of the filtercoefficient during the predetermined period; and prevent adaptation ofthe filter coefficients and decrease the gain of the amplifying means atthe end of the predetermined period.
 2. Apparatus as claimed in claim 1wherein the control means is also responsive to the ratio of the powerof signals at the first input of the echo cancelling means to the powerof signals at the output of the echo cancelling means exceeding a givenvalue to reduce the gain of the amplifying means.
 3. A repeater of a2-wire bidirectional transmission line, the repeater comprising:twounidirectional transmission paths, one for each transmission direction,each comprising echo cancelling means and substantially linearamplifying means, each echo cancelling means comprising a transveralfilter having adaptive filter coefficients and each amplifying meanshaving a controllable gain; and control means responsive to the powerratio between signals on each unidirectional transmission path at aninput to and an output of the echo cancelling means for controlling thegain of each amplifying means and for controlling adaptation of thefilter coefficients of the transversal filter of each echo cancellingmeans; wherein the control means is responsive to said power ratio ofboth unidirectional transmission paths falling below a given value to:increase the gain of both amplifying means for a predetermined period;allow adaptation of the filter coefficients of both transversal filtersduring the predetermined period; and prevent adaptation of the filtercoefficients of both transversal filters and decrease the gain of bothamplifying means at the end of the predetermined period.
 4. A repeateras claimed in claim 3 wherein, for each unidirectional transmissionpath, the control means is also responsive to said power ratio forsignals on the unidirectional transmission path exceeding a given valueto reduce the gain of the amplifying means in the unidirectionaltransmission path.
 5. A repeater as claimed in claim 4 wherein thecontrol means is also responsive to said power ratio for one of theunidirectional transmission paths exceeding a given value to preventadaptation of the filter coefficients of the transversal filter of theecho cancelling means of the other unidirectional transmission path. 6.A repeater as claimed in claim 3 wherein the control means is alsoresponsive to said power ratio for one of the unidirectionaltransmission paths exceeding a given value to prevent adaptation of thefilter coefficients of the transversal filter of the echo cancellingmeans of the other unidirectional transmission path.